Electrostatic MEMS Speakers With Embedded Vertical Actuation

In this research, micromachined silicon membranes with embedded electrostatic vertical actuator arrays capable of high out-of-plane displacement have been presented. The performance of such devices as MEMS speakers has been characterized by showing relatively high Sound Pressure level (SPL) compared to existing MEMS electrostatic speakers. Large arrays of electrostatic actuator cells, consisting of up to 10,000 cells with submicron transduction gaps, are formed on the edges of the membranes, inducing a bending moment in the membrane upon excitation. The large number of cells, along with submicron transduction gaps, allow much larger vibrational energy to be pumped into the vibrating membrane compared to the conventional electrostatic acoustic transducers, leading to higher sound output. For $50 \, \mu $ m thick membranes with a device footprint of 5mm $ \times 5$ mm, a maximum SPL of 114 dB in open air was measured at a 1 cm distance, translating to an out-of- plane displacement of over $16 \, \mu $ m for the membrane. The transducer strength figure of merit defined as acoustic pressure per membrane surface area per actuation voltage, for the tested devices, is calculated to be up to $25.1 \times 10^{-5}$ Pa/mm2/V, which is over 5X higher than the highest values calculated for the existing art. [2023-0192]